Field-aligned currents

Instead of seeing cusp signatures, Polar recorded a residual magnetic field caused by the field-aligned currents (FACs) on May 11. The magnetic field perturbation caused by FACs is basically in the east-west component, which is roughly along the Y component in GSM when Polar is near the noon-midnight meridian. In Figure 3, weak field-aligned currents (FACs) were seen as residual ${\delta}$By GSM at high altitudes around 12 UT on May 11. The same residuals continue to be present in Figure 4 when using the T96 model. This is because the large-scale FAC system that is built into the T96 model explicitly does not replicate the often narrow FACs [Zhou et al., 1997]. The changes of the magnetic field due to the FACs on May 11 are similar to those normally seen by Polar at high altitudes under northward IMF [Russell et al., 1997,Zhou et al., 1999]. The magnetic residual corresponded to a downward flowing current sheet from 80.0^o to 83^o magnetic latitude at Polar footprints and a weaker upward flowing current sheet from 77.4^o to 80.0^o magnetic latitude at Polar footprints.

The FAC signatures were also recorded by Polar at low altitudes ($\sim$ 5000 km) near the Polar perigees in the southern hemisphere on both days. There were two perigee passes on these days, and the FACs were crossed twice for each pass, one in the pre-noon sector and one in the pre-midnight sector. In Figure 5, we compare the FAC magnetic signatures seen at same local times on the two days. Shown in Figure 5 are the magnetic field residuals of the east-west component as a function of magnetic latitude of Polar footprints. The thick traces are for May 11 and the thin traces for May 14. On the nightside about two hours before midnight, the FACs showed structure of well-defined thin current sheets on both days. The upward flowing current was at higher latitude (the region 1 current) and the downward flowing current sheet at lower latitude (the region 2 current), consistent with previously determined FAC pattern [Iijima and Potemra, 1976]. On the dayside pre-noon sector, the FAC magnetic field exhibited many fluctuations and complicated structures on both days, probably caused by the motion of FACs. We note that there are no significant differences on the strength of FACs observed for different solar wind conditions on these two days. Thus, the solar wind dynamic pressure only plays a minor role, if any. The IMF Bz component is the major controlling factor of the strength of field-aligned currents. The same conclusion has been reached based on observations under very high solar wind dynamic pressure during a magnetic cloud event [Le et al., 1998].